This invention relates generally to a method of valve gated three layer injection molding and more particularly to such a method wherein the material which forms the outer layers is injected through a valve pin and the material which makes up the inner barrier layer is injected through an annular channel in the nozzle with part of the injection of both of the materials being simultaneous.
Multi-cavity injection molding apparatus for making three layer protective containers for food or preforms or parisons for beverage bottles are known. A middle layer of a barrier material such as ethylene vinyl alcohol copolymer (EVOH) or nylon is molded between inner and outer layers of a polyethylene terephthalate (PET) type material. In some cases, the two melts are injected sequentially, while in other cases both sequential injection and simultaneous coinjection are utilized. A disadvantage of these previous methods is that the middle layer of barrier material is midway between the inner and outer layers of PET. This can result in the middle layer of the barrier material cracking when a hot liquid is poured into the container.
While most of the prior art shows the barrier material being injected through a central melt channel in each heated nozzle, U.S. Pat. No. 4,717,324 to Schad et al. which issued Jan. 5, 1988 shows valve gated three layer injection molding with the PET type material being injected through the central melt channel in each nozzle. Canadian Patent Application No. 2,201,415 to Blank et al. laid open Oct. 4, 1997 also shows the PET type material being injected through a central melt bore in each valve pin. U.S. Pat. No. 5,374,178 to Nakayama which issued Dec. 20, 1994 also shows melt being injected through a central melt bore in the valve pin. However, these previous valve gated methods have the disadvantages that the two materials are injected sequentially which limits further reductions in cycle time and also produces a thin outer layer of PET.